Heat-resistant steels are employed for uses under severely corrosive conditions such as automobile exhaust emission control systems, parts of furnaces, parts of heat-exchangers, electric appliances for cooking such as electric range and grill. Such steels are required to be provided with hot gas corrosion resistance under burning conditions, hot corrosion resistance in environments containing various oxides such as PbO, V.sub.2 O.sub.5, etc., chlorides such as PbCl.sub.2, NaCl, MgCl.sub.2, KCl, etc., and resistance to hot corrosion by molten salt in addition to general high temperature characteristics such as high temperature strength, high temperature oxidation resistance, adherence of scale, etc. Further, these steels must be resistive to wet corrosion by condensed water at low temperatures. Under these severely corrosive environments, carbon steel sheets which are surface-treated for heat resistance cannot stand, and, therefore, heat-resistant austenitic stainless steels are used.
It is known and is a problem to be solved that incinerators for treating a large quantity of waste materials, tuyere burners of blast furnaces, heavy oil burners, exhaust gas pipes of internal combustion engines, etc. and parts of apparatus which are used in environments in which adhesion of salt or ash occurs such as in the cold districts where antifreezing agents are sprinkled on the roads, often suffer remarkable high temperature corrosion. Investigations of cases of this kind of corrosion have revealed that accelerated oxidation of intergranular corrosion type is observed in common in all the cases. This is a corrosion by adhering salt or molten salt and the corrosion with chlorides is marked.
For this kind of corrosion, known heat resistant stainless steels such as SUS304, SUS321, SUS310S, etc. are not satisfactory.
Generally, high degree addition of Si and Mo is effective for improvement of corrosion resistance. However, such high alloying deteriorates hot workability, reduces production yields, roughens the surface of steel in manufacturing, and makes difficult pipe-making and welding in application. Also it has been revealed that welds are often selectively and remarkably corroded by hot salt although the parent metal is sound when a fresh material is subjected to high temperatures in environments where the material comes into contact with a high concentration salt solution.
Japanese Laid-Open Patent Publication No. 63-213643 (1988) discloses a stainless steel having excellent high temperature corrosion resistance in the presence of chlorides, said steel comprising not more than 0.03% C, 10-20% Cr, 10-30% Ni, not more than 2% Mn, 1-6% Si, 0.5-5% Mo and 0.02-0.4% N, wherein the D value defined as
24.4 Cr+28 Ni+6.7 Mn-48.8 Si-56.9 Mo-148.0 Nb is not more than 500. The steel may contain 0.1-1% in total of at least one of Ti, Zr, Nb and Ta. However, improvement in weld hot cracking resistance is not considered in this steel.
Therefore, there is a demand for a heat-resistant anstenitic stainless steel which is provided with both excellent weld hot crocking resistance and excellent hot workability in addition to hot salt corrosion resistance.
We made an intensive research in order to improve hot salt corrosion resistance, weld hot cracking resistance and hot-workability of the heat-resistant austenitic stainless steel simultaneously and have found that these properties can be improved by adding limited amounts of Si and Mo for the purpose of improvement of hot salt corrosion resistance; adding a limited amount of Cu from the viewpoint of stress corrosion cracking resistance or weatherability, if desired; adding limited amounts of Nb, Ti or V for the improvement of resistance to intergranular attack hot corrosion and intergranular corrosion by combination attack in the cool state, high temperature strength as well as hot workability; and adding limited amounts of B and REM for the improvement of hot workability and weld hot cracking susceptibility.